Artist's impression of the Milky Way Galaxy. Credit: ESO
Welcome back to our Fermi Paradox series, where we take a look at possible resolutions to Enrico Fermi’s famous question, “Where Is Everybody?” Today, we examine the possibility that Earth hasn’t been visited by aliens because interstellar travel is not very practical!
In 1950, Italian-American physicist Enrico Fermi sat down to lunch with some of his colleagues at the Los Alamos National Laboratory, where he had worked five years prior as part of the Manhattan Project. According to various accounts, the conversation turned to aliens and the recent spate of UFOs. Into this, Fermi issued a statement that would go down in the annals of history: “Where is everybody?“
This became the basis of the Fermi Paradox, which refers to the disparity between high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence. Since Fermi’s time, there have been several proposed resolutions to his question, which includes the very real possibility that interstellar colonization follows the basic rule of Percolation Theory.
Artist illustration of a spacecraft passing through a wormhole to a distant galaxy. Image credit: NASA.
Special Relativity. It’s been the bane of space explorers, futurists and science fiction authors since Albert Einstein first proposed it in 1905. For those of us who dream of humans one-day becoming an interstellar species, this scientific fact is like a wet blanket. Luckily, there are a few theoretical concepts that have been proposed that indicate that Faster-Than-Light (FTL) travel might still be possible someday.
A popular example is the idea of a wormhole: a speculative structure that links two distant points in space time that would enable interstellar space travel. Recently, a team of Ivy League scientists conducted a study that indicated how “traversable wormholes” could actually be a reality. The bad news is that their results indicate that these wormholes aren’t exactly shortcuts, and could be the cosmic equivalent of “taking the long way”!
A new study considers what life could be like for civilizations 1 trillion years from now, when every star in the Universe will expand beyond the cosmic horizon. Credit: ESO/S. Brunier
It’s a staple of science fiction, and something many people have fantasized about at one time or another: the idea of sending out spaceships with colonists and transplanting the seed of humanity among the stars. Between discovering new worlds, becoming an interstellar species, and maybe even finding extra-terrestrial civilizations, the dream of spreading beyond the Solar System is one that can’t become reality soon enough!
No immediate plausibility issues with this picture, since the speedometer says 0.8c. Getting it past 1.0c is where it gets tricky.
It’s always a welcome thing to learn that ideas that are commonplace in science fiction have a basis in science fact. Cryogenic freezers, laser guns, robots, silicate implants… and let’s not forget the warp drive! Believe it or not, this concept – alternately known as FTL (Faster-Than-Light) travel, Hyperspace, Lightspeed, etc. – actually has one foot in the world of real science.
In physics, it is what is known as the Alcubierre Warp Drive. On paper, it is a highly speculative, but possibly valid, solution of the Einstein field equations, specifically how space, time and energy interact. In this particular mathematical model of spacetime, there are features that are apparently reminiscent of the fictional “warp drive” or “hyperspace” from notable science fiction franchises, hence the association.
Background:
Since Einstein first proposed the Special Theory of Relativity in 1905, scientists have been operating under the restrictions imposed by a relativistic universe. One of these restrictions is the belief that the speed of light is unbreakable and hence, that there will never be such a thing as FTL space travel or exploration.
Visualization of a warp field, according to the Alcubierre Drive. Credit: AllenMcC
Even though subsequent generations of scientists and engineers managed to break the sound barrier and defeat the pull of the Earth’s gravity, the speed of light appeared to be one barrier that was destined to hold. But then, in 1994, a Mexican physicist by the name of Miguel Alcubierre came along with proposed method for stretching the fabric of space-time in way which would, in theory, allow FTL travel to take pace.
Concept:
To put it simply, this method of space travel involves stretching the fabric of space-time in a wave which would (in theory) cause the space ahead of an object to contract while the space behind it would expand. An object inside this wave (i.e. a spaceship) would then be able to ride this region, known as a “warp bubble” of flat space.
This is what is known as the “Alcubierre Metric”. Interpreted in the context of General Relativity, the metric allows a warp bubble to appear in a previously flat region of spacetime and move away, effectively at speeds that exceed the speed of light. The interior of the bubble is the inertial reference frame for any object inhabiting it.
Since the ship is not moving within this bubble, but is being carried along as the region itself moves, conventional relativistic effects such as time dilation would not apply. Hence, the rules of space-time and the laws of relativity would not be violated in the conventional sense.
Artist’s concept of a spacecraft using an Alcubierre Warp Drive. Credit: NASA
One of the reasons for this is because this method would not rely on moving faster than light in the local sense, since a light beam within this bubble would still always move faster than the ship. It is only “faster than light” in the sense that the ship could reach its destination faster than a beam of light that was traveling outside the warp bubble.
Difficulties:
However, there is are few problems with this theory. For one, there are no known methods to create such a warp bubble in a region of space that would not already contain one. Second, assuming there was a way to create such a bubble, there is not yet any known way of leaving once inside it. As a result, the Alcubierre drive (or metric) remains in the category of theory at this time.
Mathematically, it can be represented by the following equation: ds2= – (a2 – BiBi) dt2 + 2Bi dxi dt + gijdxi dxj, where a is the lapse function that gives the interval of proper time between nearby hypersurfaces, Biis the shift vector that relates the spatial coordinate systems on different hypersurfaces and gij is a positive definite metric on each of the hypersurfaces.
Attempts at Development:
In 1996, NASA founded a research project known as the Breakthrough Propulsion Physics Project (BPP) to study various spacecraft proposals and technologies. In 2002, the project’s funding was discontinued, which prompted the founder – Marc G. Millis – and several members to create the Tau Zero Foundation. Named after the famous novel of the same name by Poul Anderson, this organization is dedicated to researching interstellar travel.
In 2012, NASA’s Advanced Propulsion Physics Laboratory (aka. Eagleworks) announced that they had began conducting experiments to see if a “warp drive” was in fact possible. This included developing an interferometer to detect the spatial distortions produced by the expanding and contracting space-time of the Alcubierre metric.
The team lead – Dr. Harold Sonny White – described their work in a NASA paper titled Warp Field Mechanics 101. He also explained their work in NASA’s 2012 Roundup publication:
“We’ve initiated an interferometer test bed in this lab, where we’re going to go through and try and generate a microscopic instance of a little warp bubble. And although this is just a microscopic instance of the phenomena, we’re perturbing space time, one part in 10 million, a very tiny amount… The math would allow you to go to Alpha Centauri in two weeks as measured by clocks here on Earth. So somebody’s clock onboard the spacecraft has the same rate of time as somebody in mission control here in Houston might have. There are no tidal forces, no undue issues, and the proper acceleration is zero. When you turn the field on, everybody doesn’t go slamming against the bulkhead, (which) would be a very short and sad trip.”
In 2013, Dr. White and members of Eagleworks published the results of their 19.6-second warp field test under vacuum conditions. These results, which were deemed to be inconclusive, were presented at the 2013 Icarus Interstellar Starship Congress held in Dallas, Texas.
When it comes to the future of space exploration, some very tough questions seem unavoidable. And questions like “how long will it take us to get the nearest star?” seem rather troubling when we don’t make allowances for some kind of hypervelocity or faster-than-light transit method. How can we expect to become an interstellar species when all available methods with either take centuries (or longer), or will involve sending a nanocraft instead?
At present, such a thing just doesn’t seem to be entirely within the realm of possibility. And attempts to prove otherwise remain unsuccessful or inconclusive. But as history has taught us, what is considered to be impossible changes over time. Someday, who knows what we might be able to accomplish? But until then, we’ll just have to be patient and wait on future research.
Inside the LHC's underground tunnel. (Credit: CERN)
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Neutrinos have been cleared of allegations of speeding, according to an announcement issued today by CERN and the ICARUS experiment at Italy’s Gran Sasso National Laboratory. Turns out they travel exactly as fast as they should, and not a nanosecond more.
The initial announcement in September 2011 from the OPERA experiment noted a discrepancy in the measured speed of neutrinos traveling in a beam sent to the detectors at Gran Sasso from CERN in Geneva. If their measurements were correct, it would have meant that the neutrinos had arrived 60 nanoseconds faster than the speed of light allows. This, understandably, set the world of physics a bit on edge as it would effectually crumble the foundations of the Standard Model of physics.
As other facilities set out to duplicate the results, further investigations by the OPERA team indicated that the speed anomaly may have been the result of bad fiberoptic wiring between the detectors and the GPS computers, although this was never officially confirmed to be the exact cause.
Now, a a statement from CERN reports the results of the ICARUS experiment — Imaging Cosmic and Rare Underground Signals — which is stationed at the same facilities as OPERA. The ICARUS data, in measuring neutrinos from last year’s beams, show no speed anomaly — further evidence that OPERA’s measurement was very likely a result of error.
The full release states:
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The ICARUS experiment at the Italian Gran Sasso laboratory has today reported a new measurement of the time of flight of neutrinos from CERN to Gran Sasso. The ICARUS measurement, using last year’s short pulsed beam from CERN, indicates that the neutrinos do not exceed the speed of light on their journey between the two laboratories. This is at odds with the initial measurement reported by OPERA last September.
What neutrinos look like to ICARUS. (LNGS)
“The evidence is beginning to point towards the OPERA result being an artefact of the measurement,” said CERN Research Director Sergio Bertolucci, “but it’s important to be rigorous, and the Gran Sasso experiments, BOREXINO, ICARUS, LVD and OPERA will be making new measurements with pulsed beams from CERN in May to give us the final verdict. In addition, cross-checks are underway at Gran Sasso to compare the timings of cosmic ray particles between the two experiments, OPERA and LVD. Whatever the result, the OPERA experiment has behaved with perfect scientific integrity in opening their measurement to broad scrutiny, and inviting independent measurements. This is how science works.”
The ICARUS experiment has independent timing from OPERA and measured seven neutrinos in the beam from CERN last year. These all arrived in a time consistent with the speed of light.
“The ICARUS experiment has provided an important cross check of the anomalous result reports from OPERA last year,” said Carlo Rubbia, Nobel Prize winner and spokesperson of the ICARUS experiment. “ICARUS measures the neutrino’s velocity to be no faster than the speed of light. These are difficult and sensitive measurements to make and they underline the importance of the scientific process. The ICARUS Liquid Argon Time Projection Chamber is a novel detector which allows an accurate reconstruction of the neutrino interactions comparable with the old bubble chambers with fully electronics acquisition systems. The fast associated scintillation pulse provides the precise timing of each event, and has been exploited for the neutrino time-of-flight measurement. This technique is now recognized world wide as the most appropriate for future large volume neutrino detectors”.
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An important note is that, although further research points more and more to neutrinos behaving as expected, the OPERA team had proceeded in a scientific manner right up to and including the announcement of their findings.
“Whatever the result, the OPERA experiment has behaved with perfect scientific integrity in opening their measurement to broad scrutiny, and inviting independent measurements,” the ICARUS team reported. “This is how science works.”
You can shelf your designs for a warp drive engine (for now) and put the DeLorean back in the garage; it turns out neutrinos may not have broken any cosmic speed limits after all.
Ever since the news came out on September 22 of last year that a team of researchers in Italy had clocked neutrinos traveling faster than the speed of light, the physics world has been resounding with the potential implications of such a discovery — that is, if it were true. The speed of light has been a key component of the standard model of physics for over a century, an Einstein-established limit that particles (even tricky neutrinos) weren’t supposed to be able to break, not even a little.
“According to sources familiar with the experiment, the 60 nanoseconds discrepancy appears to come from a bad connection between a fiber optic cable that connects to the GPS receiver used to correct the timing of the neutrinos’ flight and an electronic card in a computer,” Cartlidge reported.
The original OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment had a beam of neutrinos fired from CERN in Geneva, Switzerland, aimed at an underground detector array located 730 km away at the Gran Sasso facility, near L’Aquila, Italy. Researchers were surprised to discover the neutrinos arriving earlier than expected, by a difference of 60 nanoseconds. This would have meant the neutrinos had traveled faster than light speed to get there.
Repeated experiments at the facility revealed the same results. When the news was released, the findings seemed to be solid — from a methodological standpoint, anyway.
Shocked at their own results, the OPERA researchers were more than happy to have colleagues check their results, and welcomed other facilities to attempt the same experiment.
Repeated attempts may no longer be needed.
Once the aforementioned fiber optic cable was readjusted, it was found that the speed of data traveling through it matched the 60 nanosecond discrepancy initially attributed to the neutrinos. This could very well explain the subatomic particles’ apparent speed burst.
Case closed? Well… it is science, after all.
“New data,” Cartlidge added, “will be needed to confirm this hypothesis.”
UPDATE 2/22/12 11:48 pm EST: According to a more recent article on Nature’s newsblog, the Science Insider report erroneously attributed the 60 nanosecond discrepancy to loose fiber optic wiring from the GPS unit, based on inside “sources”. OPERA’s statement doesn’t specify as such, “saying instead that its two possible sources of error point in opposite directions and it is still working things out.”
OPERA’s official statement released today is as follows:
“The OPERA Collaboration, by continuing its campaign of verifications on the neutrino velocity measurement, has identified two issues that could significantly affect the reported result. The first one is linked to the oscillator used to produce the events time-stamps in between the GPS synchronizations. The second point is related to the connection of the optical fiber bringing the external GPS signal to the OPERA master clock.
These two issues can modify the neutrino time of flight in opposite directions. While continuing our investigations, in order to unambiguously quantify the effect on the observed result, the Collaboration is looking forward to performing a new measurement of the neutrino velocity as soon as a new bunched beam will be available in 2012. An extensive report on the above mentioned verifications and results will be shortly made available to the scientific committees and agencies.” (via Nature newsblog.)
Today, CERN announced that the LHCb experiment had revealed the existence of two new baryon subatomic particles. Credit: CERN/LHC/GridPP
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New test results are in from OPERA and it seems those darn neutrinos, they just can’t keep their speed down… to within the speed of light, that is!
A report released in September by scientists working on the OPERA project (Oscillation Project with Emulsion-tracking Apparatus) at Italy’s Gran Sasso research lab claimed that neutrinos emitted from CERN 500 miles away in Geneva arrived at their detectors 60 nanoseconds earlier than expected, thus traveling faster than light. This caused no small amount of contention in the scientific community and made news headlines worldwide – and rightfully so, as it basically slaps one of the main tenets of modern physics across the face.
Of course the scientists at OPERA were well aware of this, and didn’t make such a proclamation lightly; over two years of repeated research was undergone to make sure that the numbers were accurate… as well as could be determined, at least. And they were more than open to having their tests replicated and the results reviewed by their peers. In all regards their methods were scientific yet skepticism was widespread… even within OPERA’s own ranks.
One of the concerns that arose regarding the discovery was in regards to the length of the neutrino beam itself, emitted from CERN and received by special detector plates at Gran Sasso. Researchers couldn’t say for sure that any neutrinos detected were closer to the beginning of the beam versus the end, a disparity (on a neutrino-sized scale anyway) of 10.5 microseconds… that’s 10.5millionths of a second! And so in October, OPERA requested that proton pulses be resent – this time lasting only 3 nanoseconds each.
The OPERA Neutrino Detector
The results were the same. The neutrinos arrived at Gran Sasso 60 nanoseconds earlier than anticipated: faster than light.
The test was repeated – by different teams, no less – and so far 20 such events have been recorded. Each time, the same.
Faster. Than light.
What does this mean? Do we start tearing pages out of physics textbooks? Should we draw up plans for those neutrino-powered warp engines? Does Einstein’s theory of relativity become a quaint memento of what we used to believe?
Hardly. Or, at least, not anytime soon.
OPERA’s latest tests have managed to allay one uncertainty regarding the results, but plenty more remain. One in particular is the use of GPS to align the clocks at the beginning and end of the neutrino beam. Since the same clock alignment system was used in all the experiments, it stands that there may be some as-of-yet unknown factor concerning the GPS – especially since it hasn’t been extensively used in the field of high-energy particle physics.
In addition, some scientists would like to see more results using other parts of the neutrino detector array.
Of course, like any good science, replication of results is a key factor for peer acceptance. And thus Fermilab in Batavia, Illinois will attempt to perform the same experiment with its MINOS (Main Injector Neutrino Oscillation Search) facility, using a precision matching OPERA’s.
MINOS hopes to have its independent results as early as next year.
No tearing up any textbooks just yet…
Read more in the Nature.com news article by Eugenie Samuel Reich. The new result was released on the arXiv preprint server on November 17. (The original September 2011 OPERA team paper can be found here.)
